1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to a backlight unit and a liquid crystal display (LCD) apparatus, and more in particular, to a backlight unit in which one of s-polarized light or p-polarized light can be emitted by using wire-grid polarizer and an LCD apparatus employing the backlight unit.
2. Description of the Related Art
FIG. 1 illustrates a conventional liquid crystal display (LCD) apparatus. Referring to FIG. 1, the LCD apparatus includes an LCD panel 10 and a backlight unit 20 installed on a rear substrate of the LCD panel 10 to radiate light to the LCD panel 10. The LCD itself does not generate light and thus the backlight unit 10 provides light to produce an image. The LCD panel 10 includes a liquid crystal layer 13, transparent electrodes 12 and 14 controlling the orientation of the liquid crystal of the liquid crystal layer 13, and rear and front polarization panels 11 and 15 converting the incident light into polarized light polarized in a predetermined direction. In such a configuration, the light emitted from the backlight unit 20 is polarized in a particular direction by the rear polarization panel 11 and passes through the liquid crystal layer 13. When passing through the liquid crystal layer 13, the direction of polarization of the light is changed or not changed according to the voltage applied to the transparent electrodes 12 and 14, and the light passes through the front polarization panel 15 or is blocked by the front polarization panel 15. Thus, the pixels of the LCD apparatus are respectively turned on or off to produce an image.
FIGS. 2A and 2B are cross-sectional views of examples of the backlight unit 20. Referring to FIG. 2A, the backlight unit 20 can include a light guide plate 22 having a hologram pattern on its upper surface, a light source 21 placed at a lateral side of the light guide plate 22, and reflection panels 24 and 23 respectively placed on the lower surface and on an opposite lateral side of the light guide plate 22. Referring to FIG. 2B, the backlight unit 20 can include a light guide plate 25 with a sloped lower surface, a light source 21 placed at a lateral side of the light guide plate 25 and a reflection panel 24 attached to the lower surface of the light guide plate 25. In the backlight unit in FIG. 2A, light emitted from the light source 21 to the upper surface of the light guide plate 22 is diffracted by the hologram pattern to be emitted almost vertically, and light emitted to the lower surface of the light guide plate 22 is reflected by the reflection panel 24 and is incident on the upper surface of the light guide plate 22. In the backlight unit in FIG. 2B, light incident on the light guide plate 25 is emitted at an angle through the upper surface of the light guide plate 25 and reflected from the lower surface of the light guide plate 25 and proceeds to the upper surface of the light guide plate 25.
However, in the conventional backlight unit 20, since almost equal amounts of p-polarized light and s-polarized light are mixed and emitted, only half of the light emitted from the backlight unit is transmitted in the rear polarization panel 11 and the other half of the light is absorbed. Accordingly, the efficiency is low, and heat is generated as light is absorbed in the rear polarization panel 11. Also, there is a limit to the increase in the brightness of the LCD apparatus.
To solve these problems, the use of a dichroic polarizer sheet to emit a light having the same polarization direction as the rear polarization panel have been proposed. However, when a dichroic polarizer sheet is used, efficiency varies according to the incident angle and the wavelength of the light and light is still absorbed. A dual brightness enhancement film (DBEF) is highly efficient in that polarized light, which is not transmitted, is reflected and recycled. However, the cost is high and additional manufacturing processing is needed for the DBEF to be attached to the backlight unit.
U.S. Patent Publication No. 2003/0210369 discloses an LCD apparatus, shown in FIGS. 3A and 3B, in which brightness is improved by using a wire-grid polarizer. Referring to FIGS. 3A and 3B, the LCD apparatus includes a wire-grid polarizer 16 on a portion of a lower electrode 12.
The wire-grid polarizer 16 is made of thin metal wires disposed at regular intervals. The wire-grid polarizer 16 acts like a diffraction lattice when the distance between the metal wires is greater than the wavelength light, and acts like a polarizer when the distance between the metal wires is less than the wavelength of light. When acting like a polarizer, polarized light parallel to the metal wires, that is, s-polarized light, is reflected and polarized light perpendicular to the metal wires, that is, the p-polarized light, is transmitted.
According to the operation of the LCD apparatus disclosed in US Patent Publication No. 2003/0210369, referring to FIG. 3A, when a liquid crystal layer 13 is turned off, light emitted from a backlight unit passes through a rear polarization panel 11 and the liquid crystal layer 13 and is blocked by a front polarization panel 15. Also, external light passes through the front polarization panel 15, the liquid crystal layer 13, and the wire-grid polarizer 16, and is blocked by the rear polarization panel 11. On the other hand, referring to FIG. 3B, when the liquid crystal layer 13 is turned on, the light emitted from the backlight unit passes through the front polarization panel 15 and is emitted to the outside. On the other hand, external light passes through the front polarization panel 15, and is reflected by the wire-grid polarizer 16 as the polarization is changed by the liquid crystal layer 13, the polarization of the reflected light is changed again by the liquid crystal layer 13 and the light is emitted to the outside through the front polarization panel 15. Accordingly, not only the light emitted from the backlight unit but also the external light can be used, thereby improving brightness.
However, the brightness of the LCD apparatus cannot be improved without external light, and additional manufacturing process is needed to form the wire-grid polarizer when manufacturing LCD panels. Also, since the structure of the backlight unit is the same as in other conventional LCD panels, light is still absorbed by the rear polarization panel 11.
Accordingly, a backlight unit, which can be manufactured at low costs in a simple way without changing the structure of an LCD panel, which is relatively costly, and efficiently emit light of a particular polarization is needed.